This invention relates to materials which utilize solar energy or ambient temperature change for the storage of thermal effects, either heat or cold, utilizing the heat of fusion of the material. The invention relates more particularly to a composition of a material, and a method for producing such material, in the form of self-supporting heat exchange bodies or elements for use as heat exchangers.
With the increasing concern in regard to the exhaustion of known sources of clean energy, principally gas and oil, considerable attention has been given to the use of solar energy, and much work has been done in regard to the design of solar energy systems having the capability to store heat developed from solar energy during times of sunlight for use at other times when the sunlight is not available.
The use of heat of fusion materials, for heat storage particularly, has gained increasing favor in recent times and has been the subject of study and experimentation to develop materials which are suitable for use in practical heat exchange systems. One factor in this endeavor is to discover and develop materials which are available in large quantities, which can be made available for use at a reasonable cost, and which can be prepared for the ultimate use as heat exchangers in a simple and economic manner. The achieving of these desirable ends is not without problems; and efforts of the present applicant to solve some of the problems are described in U.S. Pat. No. 2,677,664, issued May 4, 1954 and U.S. Pat. No. 3,986,969, issued Oct. 19, 1976.
The heat of fusion materials which are the subject of this invention, are also referred to as "phase change thermal storage materials." These materials store thermal effects by cyclic changes between the solid phase and the liquid phase, with the accompanying absorption or release of heat resulting from the heat of fusion effects. A suitable example of such material is sodium sulfate decahydrate (Glauber's salt).
One particular problem with these materials results from the fact that the density of the material in the solid phase is usually higher than the density of the material in the liquid phase; and during the transformation of the material from the solid to the liquid phase certain of the solid particles of higher density will tend to sink toward the bottom of containers filled with the partially molten material. This results in a condition known as thermal stratification, in that the top layers of a container become overheated while the bottom layer remains at a much lower temperature and may remain continuously in solid or frozen form. Eventually the bottom layer will not change to the liquid phase, when the container is exposed to the upper temperature of the system, and by the same token the upper layer may not change to the solid phase when the container is exposed to the lower temperature of the system. Obviously, the heat of fusion effect and the efficiency of the system is drastically reduced. The above mentioned Telkes U.S. Pat. No. 3,986,969 is concerned with one method for solving the thermal stratification problem, involving the use of a thickener so that the material has a gel-like structure in a liquid phase to minimize settling of heavier particles.
A practical problem with respect to the construction of heat exchangers involving these heat of fusion materials, is that the material is not self-supporting while in its liquid phase; that is the material has no "load bearing mechanical strength." This means that the heat exchange assembly must be designed in a manner to provide structural support, as well as confinement, for the material when in its liquid phase. The material is, of course, self-supporting when it is in its solid or frozen phase.
The design of heat exchange assemblies would be vastly simplified, as a practical matter, if the desired phase change thermal storage material could be produced in the form of self-supporting or loading bearing unitary bodies, which bodies could be formed or cast in different shapes and sizes such as in the form of bricks or in the form of panels, for example.
Some experimentaton has been done to produce "solid building units" for heat exchange assemblies. One attempt involved the use of a porous concrete structure and utilizing vacuum methods to force a liquid phase thermal storage material into the pores. This has a disadvantage that the produced heat exchange bodies are relativey heavy; and at best only about half of such bodies are available to be filled with the heat exchange material. Another proposed method has been to enclose thickened heat exchange material in plastic bags and surround these with plastic concrete to form brick-like blocks. Disadvantages of this method are, again, the weight of the blocks and the fact that the heat exchange material would likely occupy not more than fifty percent of the volume.
Encapsulation has been attempted using relatively expensive plastic capsules to produce "microencapsulation," but because of relatively high costs this approach is not economically acceptable.
A general object of this invention is to provide heat of fusion materials which obviate the problems of thermal stratification and the lack of load bearing mechanical strength which are discussed above.
A more particular object of this invention is to provide heat of fusion materials which can be cast or formed into a rigid self-supporting body of desired configuration, and which will maintain that rigid body configuration in both the solid and liquid phases.
Another object of this invention is to provide a method for the fabrication of such heat of fusion materials in a rigid body form.
Another more particular object of this invention is to provide heat of fusion materials which include a rigid support structure, providing relatively small cells enclosing individual crystalline particles of the material thereby minimizing thermal stratification.
A further object of this invention is to provide a method for producing heat of fusion materials with the rigid cellular support structure.
Still another object of this invention is to provide a method for producing heat of fusion materials which may be cast in suitable forms to provide bodies of desired configuration, and which bodies will maintain the rigid self-supporting configuration in both the solid and liquid phases of a material.
These objects are accomplished in a body for the storage and release of thermal effects, utilizing the heat of fusion of a material forming body, consisting of: a mass of separate crystalline particles of a salt-hydrate; and a rigid, integral support structure for the mass of particles, formed by a water insoluble solid filling the voids between the particles and enclosing each particle. The particles and the support structure define a rigid, self-supporting conglomerate structure, which structure remains rigid and self-supporting for both the solid phase and the liquid phase of the salt-hydrate. More particularly, the body includes means providing a water impermeable skin completely enclosing that conglomerate structure.
The objects are accomplished, also, in a method for fabricating a heat exchange body, including a heat of fusion material, comprising the steps: preparing a mass of crystalline particles of a salt-hydrate to a generally uniform size; preparing a crust forming compound, chosen to react chemically with the crystalline particles to form a water insoluble crust surrounding each particle; thoroughly mixing the salt-hydrate particles and the crust forming compound to form a homogeneous mixture; pouring the mixture into a suitable form, and allowing the mixture to set in the form whereby the crust forming compound reacts chemically with the particles to form an integral, rigid, water insoluble crust structure surrounding each of the particles and defining an integral, cellular, support structure for said particles. More particularly, the method comprises enclosng the body with a moisture impermeable outer skin.
More particularly, the mass of crystalline particles is prepared from salt-hydrate selected from a group consisting of the chlorides, nitrates, sulfates, phosphates, and carbonates, respectively, of sodium, potassium, magnesium, aluminum, and iron; and the crust forming compound is prepared from a compound selected from a group consisting of the chlorides, nitrates, oxides, and hydroxides, respectively, of calcium, strontium and barium.
The novel features of the invention, as well as additional objects thereof, will be understood more fully from the following description.